The present invention relates to a riser connector for use in offshore well operations. In particular, the present invention relates to a riser connector for connecting a riser system to a subsea wellhead assembly. In addition, the present invention relates to a method of conducting offshore well operations using the riser connector.
Offshore operations for the exploration and production of hydrocarbons involve the use of a wellhead assembly situated on the floor of the ocean and a platform from which the operations are conducted. In the case of hydrocarbon deposits situated beneath shallow water, it is common practice to provide stationary platforms secured to the ocean floor. However, in deeper water, the use of stationary platforms becomes impractical and floating platforms are required. In use, floating platforms are connected to the blowout preventer and wellhead assemblies situated on the ocean floor by means of risers.
Currently, offshore operations are being required to be conducted in ever deeper water, necessitating the need for longer riser assemblies to connect the floating platform or vessel to the wellhead assembly on the ocean floor. However, increasing the length of the drilling riser connecting a floating platform or vessel to the assemblies on the ocean floor gives rise to a number of major problems. First, the overall weight of the riser assembly increases as the length of the riser increases. In addition, the weight of the drilling fluids or xe2x80x9cmudxe2x80x9d contained in the riser also increases as the length of the riser increases. Further, in operation, tensioning force applied by the floating platform must be maintained on the riser in order to prevent the riser from buckling. Again, the task of keeping the necessary tension on the drilling riser assembly becomes increasingly problematic as the length, and hence the weight, of the riser increases. As a result of these problems, many existing floating platforms and vessels have a maximum working depth of water in which they can safely perform downhole operations. Typically, many of these platforms and vessels are limited to operating in water up to depths of about 5000 feet. The need to operate in deeper water, for example in depths up to 10,000 feet or greater, necessitates the construction of new, larger floating platforms and vessels capable of carrying out well operations accommodating the even greater weight of risers required to operate at such depths.
As a solution to the aforementioned problems, it has been proposed to employ risers of smaller diameter than conventionally employed. Typically, offshore operations have used risers having an outer diameter of 21 inches and a nominal inner diameter of 19 inches. The proposal has been made to convert operations to a small bore riser, typically having an outer diameter of 16 inches and a nominal inner diameter of 13.5 inches. It will be readily appreciated that the reduction in diameter of the riser from 21 inches to 16 inches will result in a significant reduction in the weight of the riser assembly. The volume of a riser increases with the square of any increase in the diameter of the riser. In addition, therefore, it will also be appreciated that the weight of fluid to be retained and supported by the riser will also be significantly reduced by converting operations to a smaller diameter riser. The use of the smaller bore risers reduce the overall weight of the riser assembly, leading to savings in the loading placed on the floating platforms and vessels, reducing storage requirements, and reducing the effort required to tension the riser when in use. In principle, therefore, the use of a small bore riser would increase the working depth of many existing vessels and platforms, allowing them to operate in significantly greater depths than have been possible employing the conventional 21 inch riser.
In practice, however, converting operations to using small bore risers produces a umber of practical problems, which need to be overcome before the use of small bore risers can become feasible. The blowout preventer (BOP) stack present on the top of most undersea wellhead assemblies has a nominal internal diameter of 18.75 inches. This has become a conventional size in the design of BOP stacks, able to accommodate all the tools and equipment necessary to be passed through the wellhead assembly during subsea drilling operations. Thus, conventional BOP stacks in combination with standard 21 inch diameter risers allow the use of conventional tools, such as drill bits, cementing tools and the like. In addition, this combination accommodates the installation of standard 16 inch casings and casing hangers used in the construction of the upper portion of a well. In contrast, the small bore risers mentioned before, having an internal diameter of just 13.5 inches, will not accommodate much of the aforementioned equipment. In particular, standard drilling and cementing tools will not pass through a small bore riser. Further, it is not possible to install a 16 inch casing and its associated hangers through a small bore riser.
One solution to this problem would be to reduce the size of the wellhead assembly, including the BOP stack, and the equipment used in downhole operations. However, in addition to requiring the replacement of much of the existing equipment on platforms and vessels currently in use, this reduction in size would ultimately limit the eventual completion size of the well and its eventual production capacity. Accordingly, there is a need for a system which solves the problem of the excessive weight of the risers arising out of deep water operation, but which still allows the use of conventional downhole equipment.
One solution to this problem that has been proposed involves the use of a small bore riser, which is disconnected from the wellhead assembly at an appropriate time in order to allow for the passage of equipment of standard size. Thus, U.S. Pat. No. 4,147,221 discloses a marine riser system for use in deep water drilling operations from a floating vessel, which allows the lower end of the riser to be detached from the wellhead. The lower end of the riser is set aside to a position clear of the wellhead. A support is provided to retain the riser in the set aside position. With the riser in the set aside position, casings and tools having diameters greater than that of the riser internal diameter can then be passed into the BOP and the wellhead assembly and inserted into the well. The system of U.S. Pat No. 4,147,221 allows the use of a small bore riser, while retaining the possibility of using larger diameter equipment without the need of returning the small bore riser to the surface. However, the system of U.S. Pat. No. 4,147,221 requires the presence of support mechanisms and guidance systems in order to move the riser from the wellhead to the set aside position. In addition, means and methods must be provided in order to adjust the rig-applied tension during the riser""s movement between the wellhead and the set aside support position.
An alternative design of set aside system is disclosed in International patent publication number WO 00/34618. The system uses a reduced diameter drilling riser, in turn reducing the size and cost of the attendant floating platform or vessel. The system has means for disconnecting the riser from the BOP stack on the wellhead assembly and repositioning it in a set aside position. The system comprises a mud return assembly to which the riser is connected in the set aside position. Larger diameter tools and casings can be passed through the BOP and wellhead into the well and downhole operations conducted with drilling fluids being returned through the mud return assembly and the reduced diameter riser. The riser may be returned to the BOP and wellhead assembly and normal operations resumed, once the operations requiring the larger diameter equipment have been completed. While the system of WO 00/34618 allows the riser to be used as the return path for the drilling fluids when in the set aside position, it still requires the riser to be disconnected from the BOP and wellhead assembly and moved aside. Again, the system requires means for guiding and tensioning adjustment of the riser assembly, while it is being moved between the set aside position and the wellhead.
U.S. Pat. No. 6,367,554 B1 discloses a further improved system for employing a small bore riser. The riser may be disconnected from the wellhead assembly when it is required to gain access to the BOP and wellhead assembly and into the well with larger diameter equipment and tools. The system includes a stress member to which the riser is mounted. The stress member is flexible and is deflected by a shifter to move the riser out of connection with the BOP and wellhead assembly. The stress member acts as a conduit to allow a hydrostatic head to be maintained to control the welt In a preferred embodiment, the stress member includes choke and kill lines which remain in connection throughout the operation. In this way, the small bore riser may be moved aside with the minimum of disruption to the operation of the well with no actual structural disconnect involved.
U.S. Pat. No. 3,139,932 discloses a wellhead with a tool diverter for use in so-called xe2x80x9cthrough-the-flow-linexe2x80x9d well maintenance techniques. The wellhead is in the form of a Y-shaped tube assembly and comprises a diverter to allow tools introduced through the line to be selectively passed along one branch of the Y-shaped tube. In this way circulating objects pumped through the tubing string may be selectively diverted and discharged through the appropriate orifice of the wellhead. U.S. Pat. No. 3,139,932 is silent about offshore downhole operations employing risers, in particular small bore risers, and the use of larger diameter equipment and tools.
It will be appreciated that, while the set aside systems proposed in the prior art offer a solution to many of the problems associated with the use of small bore risers, it is by no means a simple operation to disconnect the small bore riser from the wellhead and move it aside, in order to gain access to the BOP and wellhead assembly with equipment and tools of larger diameter than can be accommodated by the smaller riser. The riser must be maintained under tension during the entire set aside operation. In addition, it is necessary to provide support and power to the mechanism used to move the riser during the set aside operation. Further, flexible hoses and pipes must be employed in order to maintain the service lines, including choke and kill lines, in operation during the set aside. These requirements add to the size and complexity of the apparatus required to carry out these procedures. Accordingly, there is a need for an improved system allowing for the use of small bore risers, while permitting larger diameter equipment, in particular casings and casing hangers, which cannot be accommodated by the small bore riser to be installed with a minimum of disruption to normal operations.
There has now been found a connector assembly for connecting a riser to a wellhead assembly, which allows a riser to be connected to the subsurface wellhead and allow normal well operations to be carried out using the riser, while at the same time allowing access to the wellhead assembly for other tools and equipment using open water techniques, without requiring the riser to be disconnected from the wellhead assembly. The riser connector is particularly useful for connecting a small bore riser to a subsurface wellhead assembly allowing well operations to be carried out using the small bore riser, while also allowing access to the wellhead assembly for larger diameter tools and equipment not able to be accommodated by the small bore riser without requiring the riser to be disconnected from the wellhead assembly or set aside.
According to the present invention, there is provided in a first aspect a riser connector for connecting a riser to a wellhead assembly having a bore therethrough, the connector comprising: a housing having a first and a second end; the first end of the housing having an opening of a diameter at least substantially the same as the inner diameter of the bore of the wellhead assembly and being adapted for connecting to the wellhead assembly with the opening in communication with the bore of the wellhead assembly; the second end comprising a first opening connectable to the riser and having a diameter at least substantially the same as the inner diameter of the riser, and a second opening, having a diameter at least substantially the same as the diameter of the bore of the wellhead assembly; the housing further comprising a first conduit connecting the opening in the first end of the housing to the first opening in the second end of the housing, and a second conduit connecting the opening in the first end of the housing to the second opening in the second end of the housing.
The first opening in the second end of the connector preferably has an inner diameter less than the diameter of the bore of the wellhead assembly. In this way, the riser connector allows a small bore riser to be connected to the wellhead by means of the first conduit such that well operations, for example drilling, may be conducted through the riser in the conventional manner. When the need arises to carry out well operations which cannot be accommodated by the small bore riser, the second conduit is available to access the wellhead assembly and the well itself Preferably, the well operations gaining access to the wellhead through the second conduit will be conducted using open water techniques known in the art. If desired, however, a riser of conventional size can be lowered into place and connected to the second opening in the second end of the housing for the duration of the well operations requiring a larger diameter riser assembly.
The first and second conduits may be substantially separate from one another and discrete within the housing. Alternatively, a portion of the first and second conduits may occupy the same volume within the housing. In one embodiment, the housing defines a chamber extending from the first end to the second end. The first and second conduits occupy portions of the chamber.
The riser connector preferably comprises one or more guides, by which tools and equipment being passed through the connector are guided through the opening in the first end of the housing and into the wellhead assembly. Most conveniently, the one or more guides may be formed from a portion of the housing adjacent the first end of the housing.
In one embodiment of the present invention, the first conduit has a central longitudinal axis lying on a substantially straight line extending between the center of the opening in the first end of the housing and the first opening in the second end of the housing. In this way, when the connector is in normal use, tools and equipment may be passed from the riser through the first conduit in the riser connector without substantial deviation. It is preferred that the riser connector is so arranged that, when the connector is mounted on a wellhead assembly and in use, the central longitudinal axis of the first conduit is substantially coaxial with the central longitudinal axis of the central bore of he wellhead assembly. In this way, tools and equipment can pass down the riser, through the connector and into the wellhead without needing to overcome a substantial change in direction.
In this first embodiment of the riser connector, the second conduit may extend from the opening in the first end of the housing to the second opening in the second end with its central longitudinal axis forming a substantially straight line between the center of the opening in the first end of the housing and the center of the second opening in the second end of the housing. In this arrangement, the longitudinal axis of the second conduit is angled to the longitudinal axis of the first conduit. The riser connector may be sufficiently long that the angle between the longitudinal axes of the first conduit and the second conduit is sufficiently small that, in use, tools and equipment introduced into the wellhead assembly through the second conduit are subjected to only a minor directional change.
When the connector of this embodiment is in use, advantage may be taken of the flexible joint present in many conventional LMRP""s to vary the orientation of the riser connector. While well operations using the riser to gain access to the wellhead assembly through the first conduit are in progress, the riser connector and that portion of the wellhead assembly above the flexible joint can be oriented such that the longitudinal axis of the first conduit is substantially coincident with the central longitudinal axis of the wellhead assembly. When it is required to conduct well operations through the second conduit, the riser connector and the portion of the wellhead assembly above the flexible joint may be moved to allow the longitudinal axis of the second conduit to be substantially coincident with the central longitudinal axis of the wellhead assembly.
Alternatively, the riser connector of the present invention may be prepared from a material flexible enough to allow the connector itself to be oriented to bring the first and second conduits into substantial alignment with the bore of the welfhead assembly as described above. In this way, the riser connector itself provides flexibility between the wellhead and the riser, thus eliminating the need for a flexible joint.
In a second embodiment of the riser connector of the present invention, the second conduit is formed so as to have a curved longitudinal axis. The radius of curvature of the longitudinal axis of the second conduit is selected such that the deviation in direction of tools and equipment passing through the second conduit during well operations can be accommodated. Alternatively, the second conduit may comprise a plurality of straight portions angled such that the second conduit extends from the opening in the first end to the second opening in the second end of the housing. Again, the number and angle of the portions of the second conduit are selected such that the deviation of tools and equipment passing through the second conduit may be accommodated during normal well operations.
In use, the second embodiment of the riser connector may take advantage of the flexible section present in many LMRP""s to vary the orientation of the riser connector as described above. Alternatively, the riser connector itself may be formed from a flexible material, enabling the connector to be flexed and vary the orientation of the first and second conduits. In this respect, it is to be understood that references to variations in the orientation of the riser connector include both rotational movement, as well as side-to-side movement of the connector.
In some cases, in particular when using a small bore riser and conducting well operations using larger diameter equipment through the open water, it may be preferred to provide the riser connector with a second conduit that has a longitudinal axis extending in a straight line from the center of the opening in the first end of the connector and the center of the second opening in the second end. The riser connector can thus be oriented above the wellhead assembly such that the longitudinal axis of the second conduit is substantially vertical and coincident with the axis of the bore in the wellhead assembly. The first conduit will then have a straight or curved longitudinal axis angled to the longitudinal axis of the first conduit and the bore of the wellhead assembly. The larger tools and equipment being used during the second well operations are less able to accommodate and negotiate deviations in the passage into the well. With such an arrangement, these larger diameter tools will require little or no deviation when being passed into the well. The first conduit will comprise some minor deviations in the direction of equipment passing through it and the wellhead assembly into the well. However, the smaller diameter tools and equipment being passed through the small bore riser are more able to negotiate deviations in the path through the first conduit of the connector and into the wellhead assembly.
In embodiments of the present invention in which, in use, the orientation of the riser connector is to be varied, the riser connector preferably comprises an actuator to act upon the connector and vary its orientation. In such cases, the connector is preferably biased to the orientation in which the longitudinal axis of the first conduit is aligned with the central longitudinal axis of the wellhead assembly. However, in situations mentioned above in which larger diameter tools and equipment are being passed through the open water into the second conduit, it may be preferably to bias the connector such that the longitudinal axis of the second connector is aligned with the central longitudinal axis of the wellhead assembly.
Preferably, the riser connector of the present invention comprises a valve in the second conduit, by which the second conduit may be closed. Further, a valve may be provided by which the first conduit may be closed. In this way, in use, the riser may be isolated and the drilling fluids present in the riser prevented from escaping during well operations using the second conduit. In a preferred embodiment, a valve is provided, for example a sliding gate valve, by which either one of the first and second conduits or optionally both conduits may be closed. In this way, well operations may be conducted using either conduit to gain access to the wellhead assembly with the option of keeping the other conduit open, for example for the supply of drilling fluids to or from the well.
The riser connector of the present invention may be connected directly to the wellhead assembly. However, the riser connector may be placed at any suitable point between the surface vessel or platform and the subsea wellhead assembly.
In a further aspect, the present invention provides a wellhead assembly, comprising a central bore through the wellhead assembly, and a riser connector, the riser connector comprising: a housing having a first and a second end; the first end of the housing being connected to the wellhead assembly and having an opening in communication with the bore of the wellhead assembly, the opening having a diameter at least substantially the same as the bore of the wellhead assembly; the second end comprising a first opening connectable to a riser and having a diameter at least the same as the inner diameter of the riser, and a second opening, having a diameter at least substantially the same as the diameter of the bore of the wellhead assembly; the housing further comprising a first conduit connecting the opening in the first end of the housing to the first opening in the second end of the housing, and a second conduit connecting the opening in the first end of the housing to the second opening in the second end of the housing.
The wellhead assembly may be of conventional design and may comprise a blowout preventer (BOP) stack, as commonly employed in the art. However, the connector of the present invention is not limited in its use to wellhead assemblies comprising a BOP stack and may be equally well employed in combination with wellhead assemblies without BOP stacks.
The riser connector used in the wellhead assembly may have the features described above. The wellhead assembly may comprise a flexible joint positioned so as to lie between the well and a riser connected to the wellhead assembly during use. As noted above, the riser connector of the present invention may be flexible. Accordingly, the riser connector may act as the flexible joint in the wellhead assembly.
The wellhead assembly is of particular use in conjunction with a small bore riser, which may be connected to the wellhead assembly by means of the riser connector, allowing access from the end of the riser to the well by means of the first conduit in the riser connector.
In a further aspect, the present invention provides a method of conducting well operations on an offshore well having a wellhead assembly, the wellhead assembly having a bore, the method comprising: providing a first conduit having an upper end and a lower end, the lower end being in communication with the bore of the wellhead assembly; connecting a riser to the upper end of the first conduit; providing a second conduit having an upper end and a lower end, the lower end being in communication with the bore of the wellhead assembly, the upper end being accessible through the open water; conducting first well operations accommodated by the inner diameter of the riser through the riser and first conduit; conducting second well operations through the open water and the second conduit.
It is a particular advantage of the method that the riser need not be disconnected from the first conduit while the second downhole operations are performed.
The method may employ a small bore riser connected to the upper end of the first conduit, through which the first well operations are conducted. The method is particularly useful for the installation in the well of apparatus having an outer diameter greater than the inner diameter of the small bore riser. Such apparatus includes casings, drilling tools, completion tools and the like.
It is a further advantage of the method of this invention that the apparatus, such as casings, may be prestaged in the open water alongside the riser, be it of conventional bore or a small bore riser, while the first well operations are in progress. It is a simple operation to interrupt the first well operations to set the apparatus through the second conduit at the appropriate time without disconnecting the riser from the first conduit.
The wellhead assembly may comprise a flexible joint, as discussed above. The upper end of the second conduit may be located above the flexible joint and the tools and equipment, such as casings and casing hangers, introduced into the well through the upper end of the second conduit above the flexible joint.
The second conduit will generally remain closed while the first well operations are performed through the riser and the first conduit. The first conduit may be closed while the second well operations are performed. Alternatively, the first conduit may remain open while the second well operations are performed through the open water and the second conduit. This allows the riser and the first conduit to be of use, for example for the supply or return of muds or other drilling fluids to or from the well.
The riser connector may comprise an additional opening, for example in the housing, which may be adjacent the first or second end of the connector. This additional opening, typically sealed by a valve, will allow the contents of the riser to be evacuated, if this proves necessary.
As noted above, the riser connector and method of the present invention allow open water operations to gain access to the bore of a well through the wellhead assembly above the flexible joint. Accordingly, in a further aspect, the present invention provides a method of running a casing in the bore of an offshore well, the well having a wellhead assembly comprising a flexible joint and connected to the surface by a riser, which casing has an outer diameter greater than the inner diameter of the riser, the method comprising providing the casing through the open-water and accessing the wellhead assembly through an opening located above the flexible joint.
Specific embodiments of the apparatus and method of the present invention will now be described in detail having reference to the accompanying drawings. The detailed description of these embodiments and the referenced drawings are by way of example only and are not intended to limit the scope of the present invention.